Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name.
This compound is a dark-purple solid under anhydrous conditions, bright orange when wet and which dissolves in water concomitant with hydrolysis. Millions of kilograms are produced annually, mainly for electroplating. Chromium trioxide is a powerful oxidiser and a carcinogen.
Approximately 100,000 tonnes are produced annually by this or similar routes.
The solid consists of chains of tetrahedrally coordinated chromium atoms that share vertices. Each chromium center therefore shares two oxygen centers with neighbors. Two oxygen atoms are not shared, giving an overall stoichiometry of 1:3.
Chromium trioxide is mainly used in chrome plating. It is typically employed with additives that affect the plating process but do not react with the trioxide. The trioxide reacts with cadmium, zinc, and other metals to generate passivating chromate films that resist corrosion. It is also used in the production of synthetic rubies. Chromic acid solution is also used in applying types of anodic coating to aluminium, which are primarily used in aerospace applications. A chromic acid/phosphoric acid solution is also the preferred stripping agent of anodic coatings of all types.
^Stephens, J. S.; Cruickshank, D. W. J. (1970). "The crystal structure of (CrO3)∞". Acta Crystallographica Section B. 26 (3): 222. doi:10.1107/S0567740870002182.
^Zhai, H. J.; Li, S.; Dixon, D. A.; Wang, L. S. (2008). "Probing the Electronic and Structural Properties of Chromium Oxide Clusters (CrO 3)− n and (CrO3)n (n = 1–5): Photoelectron Spectroscopy and Density Functional Calculations". Journal of the American Chemical Society. 130 (15): 5167. doi:10.1021/ja077984d.